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Figure 18.5. LEUART Local Loopback
LOOPBK = 0
μC
LEUART
LOOPBK = 1
μC
LEUART
TX
RX
LEUn_TX
LEUn_RX
TX
RX
LEUn_TX
LEUn_RX
18.3.7 Half Duplex Communication
When doing full duplex communication, two data links are provided, making it possible for data to be
sent and received at the same time. In half duplex mode, data is only sent in one direction at a time.
There are several possible half duplex setups, as described in the following sections.
18.3.7.1 Single Data-link
In this setup, the LEUART both receives and transmits data on the same pin. This is enabled by setting
LOOPBK in LEUARTn_CTRL, which connects the receiver to the transmitter output. Because they are
both connected to the same line, it is important that the LEUART transmitter does not drive the line when
receiving data, as this would corrupt the data on the line.
When communicating over a single data-link, the transmitter must thus be tristated whenever not
transmitting data. If AUTOTRI in LEUARTn_CTRL is set, the LEUART automatically tristates LEUn_TX
whenever the transmitter is inactive. It is then the responsibility of the software protocol to make sure
the transmitter is not transmitting data whenever incoming data is expected.
The transmitter can also be tristated from software by configuring the GPIO pin as an input and disabling
the LEUART output on LEUn_TX.
Note
Another way to tristate the transmitter is to enable wired-and or wired-or mode in GPIO.
For wired-and mode, outputting a 1 will be the same as tristating the output, and for wired-
or mode, outputting a 0 will be the same as tristating the output. This can only be done on
buses with a pull-up or pull-down resistor respectively.
18.3.7.2 Single Data-link with External Driver
Some communication schemes, such as RS-485 rely on an external driver. Here, the driver has an extra
input which enables it, and instead of Tristating the transmitter when receiving data, the external driver
must be disabled. The USART has hardware support for automatically turning the driver on and off.
When using the LEUART in such a setup, the driver must be controlled by a GPIO. Figure 18.6 (p. 228)
shows an example configuration using an external driver.
Figure 18.6. LEUART Half Duplex Communication with External Driver
μC
GPIO
LEUART
TX
RX
18.3.7.3 Two Data-links
Some limited devices only support half duplex communication even though two data links are available.
In this case software is responsible for making sure data is not transmitted when incoming data is
expected.
2011-04-12 - d0001_Rev1.10
228
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